You only get one download of any movie purchased on the PSN Store. If something happens, like a corrupted hard drive, or my little mistake as described in this post, you can't download the movie again.
Straight from Playstation help pages:
"Content cannot be re-downloaded once it has been downloaded to either a PLAYSTATION 3 or PSP system"
http://playstation.custhelp.com/cgi-bin ... dmll&p_li=
I've read that it's possible to get another download, just once, by contacting support, but can't personally confirm that. The couple of movie purchases I made are gone, can't be re-downloaded, and I'm not going to bother contacting Sony support to try and get them again.
I'm sure many other people feel the same way, judging by the comments on the Official Playstation blog:
http://blog.us.playstation.com/2009/02/ ... /#comments


